This project is concerned with the relation of the structure to the function of dihydrofolate reductase from Methotrexate (MTX)-resistant murine cells, chicken liver, and a MTX-resistant subline of human lymphoblastoid cells. Using several techniques of protein chemistry, the problems to be investigated include: (a) equilibrium binding studies of substrates, coenzymes and inhibitors to enzyme using circular dichroic and fluorometric techniques; (b) chemical modification of specific amino-acid residues on the protein and measurement of the effect of these modifications upon the binding of substrates and inhibitors and upon enzymic activity; (c) amino-acid sequence determination of enzymes from Walker-256 carcinoma and MTX-resistant lines of murine and human lymphoblastoid cells; (d) examination of inhibition properties of selected quinazoline and triazine derivatives and other compounds towards dihydrofolate reductases; (e) determination of transport characteristics of selected quinazoline drugs in L1210 and in human lymphoblastoid cell lines; (f) design and evaluation of active site-directed irreversible inhibitors of dihydrofolate reductase.